This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Water content in the lungs is critical to human life. Proper pulmonary circulation of blood is very important in maintaining adequate gas exchange. The pulmonary vessels arrive from the right ventricle and divide into branches forming thin sheets of capillary network in the alveolar wall. Gas exchange occurs by diffusion between these thin capillaries and alveolar membranes. Continuous transudation of fluid in the alveolar interstitial space from the alveolar capillaries and extra-alveolar arterioles occurs normally. The fluid supplies nutrients to the lung tissue and is reabsorbed at the venular end of the capillaries or by lymphatic system.
Excess amounts of fluid may accumulate in the alveolar or lung tissue and interfere with gas exchange. Accumulation of excess water is referred to as alveolar or pulmonary edema. The four major pathophysiologic mechanisms for development of alveolar edema are: 1)increased hydrostatic pressure e.g. left heart failure 2)decreased oncotic pressure e.g. overhydration, nephrosis 3)increased capillary permeability e.g. infection, exposure to toxic substances and 4)obstruction of the lymphatic system.
Alveolar or pulmonary edema often manifests as difficulty in breathing, diaphoresis and cough. On examination patients may appear anxious, with presence of tachycardia, cyanosis, pulmonary rales, rhonchi and/or wheeze and arterial hypoxemia. The pulmonary capillary wedge pressure may be elevated and chest x-ray may show vascular redistribution, blurriness of vascular outlines, increased interstitial markings and/or butterfly pattern characteristic of alveolar edema. The clinical presentation may not be clear especially in unconscious patients. In some clinical situations such as sepsis, lung injury or pneumonia the clinical presentation may overlap. The pulmonary capillary wedge pressure used for diagnosis and monitoring of pulmonary edema is an invasive procedure and may not be an effective diagnostic and monitoring tool in some situations where the pulmonary edema is secondary to non-cardiac etiologies.